The present invention relates to a scroll vacuum pump and especially to a scroll vacuum pump in which a fixed wrap of a fixed scroll is engaged with an orbiting wrap of an orbiting scroll rotatably mounted to an eccentric portion of a driving shaft in a housing, the orbiting scroll being revolved at a certain eccentricity by the driving shaft so that a gas sucked through the outer circumference of the housing is compressed towards the center and discharged.
The scroll vacuum pump is known. In the scroll vacuum pump, operation time is too long, and the temperatures of the eccentric portion of the driving shaft and bearing and packing for supporting it are raised to cause the bearing and packing to be damaged, so that they become difficult to be used.
Thus, in the scroll vacuum pump, to increase durability, it is essential not to become excessive high temperature on the eccentric portion of the driving shaft even when operation time is too long.
To comply with the demand, the following measures are known among persons skilled in the art:                (1) Low-temperature or room-temperature air or nitrogen is introduced into a compressing region of the scroll vacuum pump and cools the region while diluting a toxic gas in the compressing region.        (2) A ventilating hole formed axially in the driving shaft allows low-temperature or room-temperature air or nitrogen to be discharged onto the bearing and to be introduced into the compressing region. After cooling to the region it is discharged.        (3) The eccentric portion of the driving shaft is hollow and low-temperature or room-temperature air is introduced into the eccentric portion to cool it.        
JP 63-105294A discloses that an inert gas such as N2 is introduced from the outside as the above (1) and that FIG. 5 thereof illustrates the gas flowing path in the eccentric portion of the driving shaft as the above (3).
However, there are problems in the technical measures. To introduce a low-temperature or room-temperature air or nitrogen into the compressing region, it is necessary for an introducing path to be provided therein and for means for supplying the gas to be provided outside. So its structure becomes complicate and larger to increase cost.
When the driving shaft rotates, the low-temperature or room-temperature air or nitrogen is introduced through the ventilating hole of the driving shaft to cool the bearings, but when the driving shaft stops, a toxic gas or impurity-including gas in the compressing region runs back and is discharged to the outside via the ventilating hole to cause environmental pollution.